Semiconductor manufacturing consists of a complex set of processes. These processes include several building blocks, e.g.: (i) deposition of thin and thick films of material on a substrate, (ii) applying a patterned mask on top of the films by photolithographic imaging, and (iii) etching the films selectively to the mask. Each additive layer contributes to the wafer bow along with thermal processing. Also, even subtractive etch processes contributes to the wafer bow.
In the manufacturing of semiconductor devices, a wafer (e.g., silicon, device, or product) is placed on a wafer handler such as glass, bare silicon, etc. for processing of the product wafer. These processes include, for example, wafer thinning, as well as formation of structures on the wafer, e.g., deposition and patterning of films to form wiring, transistors, vias, metal pads, solder bumps, chip to chip interconnects, etc.
Prior to thinning, a product wafer has its own structural integrity and has a bow within the range of +/−300 microns which can be handled by most tools. Thinning the wafer eliminates this structural integrity and mandates the use of a handler wafer (e.g., glass, bare silicon). The handler bow and the thinned wafer bow together will combine to give the overall bow of the bonded pair. After thinning of the wafer to expose through silicon vias, for example, this bow can be well in excess of +/−300 microns. This bow can be even larger when using Si wafers from more advanced technology nodes.
It has been found through standard semiconductor manufacturing processes, bonded and thinned wafers on a wafer carrier can exceed process tool wafer handler capabilities and bow limits. This, in turn, can result in wafer mis-handling, tool errors, and excessive wafer breakage during wafer finishing process steps. By way of example, it has been found that thinning of the wafer results in fluctuations in wafer handler bow. Also, through wafer finishing, the deposition of patterned metal, dielectric, and polymer layers on the grind side of the wafer exert additional stress on the bonded thinned wafer causing further fluctuations in the bow of the bonded pair (and hence wafer). Further, during thermal processes the glass transition temperature of the bonding adhesive can be exceeded, softening the adhesive and resulting in movement of the thinned wafer and wafer handler apart from each other. Once the bonded pair cool and the adhesive returns to a rigid state, the thinned wafer and the wafer handler will have a new bow. This new bow, though, may be too excessive for wafer handling downstream.
Handling of the wafer handler itself in steps prior to joining with the product wafer can be an issue as the wafer handler can be engineered to have bow in excess of its inherent bow. Glass wafer handlers alone do not have the appropriate strength and rigidity to support a thinned wafer through semiconductor manufacturing processes.